The semiconductor laser element is used as a small, inexpensive laser source such as one for the optical pickup to read data optically from optical disks. As shown in FIG. 8, a laser beam is outputted illustratively from an area 28 about 2 .mu.m wide on an end surface 26 of a semiconductor laser element 10 having an active layer about 0.1 .mu.m thick. As indicated, the light-emitting area 28 is shaped substantially as a rectangle, one pair of sides of which is longer than the other. Since light is diffracted more at the shorter sides than at the longer sides of the rectangle, the output laser beam has a cross section which is not a true circle but an ellipse elongated in the direction of the shorter sides. To use such an output laser beam as the light source of an optical pickup requires adjusting the laser beam so that the beam will have a true circle in shape and will have parallelism. The requirement is met generally by means of lens-based optics.
Lenses are made from plastics or glass. Glass lenses are preferred because plastic lenses, though easy to machine, are prone to degrade in characteristics due to expansion or contraction provoked by the heat of the beam.
Today, optics design is carried by computers running optical system (optimizing) design programs. The procedure makes it relatively easy to obtain optical systems with desired characteristics. However, optimally designing an optical system does not necessarily make the actual production of the designed system implementable because of technical or cost constraints.
A plurality of spherical lenses are usually employed to build an optical system for making the output beam of a semiconductor laser element into one which is a true circle in shape and has parallelism. The multiple lenses tend to constitute a bulky optical system. To implement an optical system of the same characteristics with fewer lenses requires the use of an aspherical lens arrangement. Manufacturing an aspherical lens from glass is an expensive process involving sophisticated machining techniques. In particular, it is very difficult to accomplish the precise machining of free curves on lenses whose diameters are less than one mm.
Semiconductor laser elements are known to have variations in characteristics therebetween. Thus if the optical system is designed, using numerous semiconductor laser elements, to obtain desired optical characteristics, said characteristic variations between the elements can lead to a lower yield of the products.
Under the circumstances, a need has been recognized for easy manufacturing of a small optical element (lens) that adjusts an output laser beam of a semiconductor laser element as desired one, e.g., into a beam which is a true circle in shape and has parallelism.
The optical system may also be reduced in size by outright elimination of a small optical element to make way for a novel optical setup such as a semiconductor laser element mounted with a lens having desired optical characteristics. There is clearly a need for such a lens-mounted semiconductor laser element. Also desired earnestly are a device and a method for machining such an optical element with relative ease.
Meanwhile, high-power semiconductor laser elements among the semiconductor laser elements serve as small, highly efficient laser sources. They are utilized as excitation sources of laser-based machine tools and other laser media.
Attempts to gain a laser beam of higher power can destroy the laser element if its active layer (i.e., stripe width) is not sufficiently thick. Thus, common practice to minimize optical damage to the element involves widening its stripe width illustratively to about 100 .mu.m. The trouble is that greater stripe widths are liable to invoke a multiple-peak mode corrupting the single-peak characteristic of the laser beam. In other words, the output area 28 of the end surface 26 can be seen apparently to output a plurality of laser beams.
As described, high-power semiconductor laser elements with extended stripe widths of their active layers are thus prone to multiple peaks in the output laser beam; laser beam coherence is not available. However, a high-power semiconductor laser element will offer a single-peak mode if its resonant mirror-finished surface (light-emitting end surface) is a concave instead of plane. Such a high-power semiconductor laser element will drastically enhance the efficiency of laser beam utilization.
To obtain desired optical characteristics, it is necessary to carry out a process of precisely machining, on the order of microns, the end surface of the element acting as the resonant mirror-finished surface of the high-power semiconductor laser element. That process is delicate and is difficult to implement.
As mentioned, semiconductor laser elements tend to have variations in characteristics therebetween. Given such a tendency, measures need to be taken to improve the yield percentage in producing numerous semiconductor laser elements having the same desired optical characteristics.
First to be desired is a semiconductor laser element that generates a single-peak output laser beam. Also desired is a semiconductor laser element whose optical characteristics may be adjusted to make the output laser beam into one which is a true circle in shape and has parallelism. Naturally, it is desired to have a device and a method for easily machining laser elements with a high precision so that they will possess the above necessary optical characteristics.
It is therefore an object of the present invention to provide a lens machined to offer the desired optical characteristics, a semiconductor laser element which is mounted with such a lens, a semiconductor laser element which has its light-emitting surface machined to provide the desired optical characteristics, and a device and a method for precisely machining the lens and element to produce the desired optical characteristics.